Such apparatus for the transmission of a teaching program in a classroom is known. It is sold under the designation "Sprachlabor". It requires the classroom to be provided with a number of special installations. For the teacher there is installed a special cabinet which is usually separated from the rest of the classroom by a glass wall. This brings about a separatation of the students and the teacher which in many instances has proved quite unfavorable pedagogically because it isolates the teacher from the students. If the student receiving devices are connected with the teaching apparatus by cable, the entire room must be wired which has the result that not only installation cost is high but also that the desks in the room can no longer be moved from one place to the other. The practically permanent installation despite plug-in connections interferes with use of the room for other purposes and makes difficult daily cleaning of the room. A further objection is limited flexibility particularly in "multimedia" instructions. By reason of these complexities of the system, this initially expensive equipment is rarely used in normal high schools in which it is installed because the instructor finds the manipulation of many technical operating elements too difficult in addition to his teaching activity.
Efforts to provide wireless teaching equipment with which the transmission is by radio have led to almost unsolvable problems. There are available only very few channels in the frequency spectrum for such transmission. The installations require a license. Good electrical isolation is required in the classrooms in which such equipment is installed. If two or more classrooms are provided with such equipment, it is necessary to provide shielding so that signals transmitted in one classroom do not disturb a neighboring classroom. Tuning of apparatus used in a second room to another frequency is not always possible because of the limited number of commercial frequencies available. Moreover, if different frequencies are used, this results in the disadvantage that the apparatus for one room is not interchangeable with that of the other room because it is incompatible. Because of the different numbers of students in the individual rooms, such interchange of equipment from one room to another is desirable. Apparatus with frequency selection switching is even more expensive to purchase.
For these reasons teaching equipment has been installed in which the electrical signals from the teachers cabinet to the students apparatus are transmitted by fixed wiring. However, this involves a high installation cost. The electrical wiring is expensive, makes shifting of the work tables impossible and is also, to a certain degree, reason for further susceptibility to failure of the complicated equipment.
Both of these systems have the common disadvantage that their use in a school requires movement of classes out of their classrooms to the room provided with the teaching installation. Such movement brings about a great disturbance in the school so that many schools on account of the above-indicated disadvantages and this further disadvantage decide not to purchase a "Sprachlabor".